Surface wave transducer, especially of split finger design, with suppression of reflections of terminal transducer fingers

ABSTRACT

A surface wave transducer has measures for suppressing internal reflections on terminal fingers of a transducer having other transducer fingers, which are arranged interdigitally adjacent to the terminal fingers, which are arranged corresponding to a specified acoustic wavelength of the transducer at intervals specified by a predetermined scheme and which are designed with finger widths that are likewise specified by the predetermined scheme. On each end of the transducer only a plurality of terminal fingers, which are connected to one busbar of the transducer, are positioned deviating from the scheme for positionings and finger widths of the other transducer fingers and are positioned deviating in their finger strip width. Phases and amplitudes of wave components, which would otherwise be reflected by the terminal transducer fingers and by an adjacent final transducer finger of the other transducer fingers, which corresponds to the scheme of the transducer, are modified such that the wave components intrinsically interfere destructively with one another.

BACKGROUND OF THE INVENTION

The present invention relates to a surface wave for suppressingreflections of terminal transducer fingers transducer.

In order to suppress undesired reflections of the acoustic wave in thetransducer and on its transducer fingers, it has been known for a longtime for such (transmitting/receiving) transducers to be constructedwith interdigitally arranged transducer fingers, especially as splitfinger transducers. In comparison with a simple interdigital transducerwith adjacent transducer fingers having alternating polarity, suchtransducers have in each case two (if required, also in each case three)adjacent fingers which have the same polarity, that is to say they areelectrically connected to one and the same busbar. In a normal, regulardesign of such a split finger transducer, there follow alternating withone another two (or three) transducer fingers connected to the onebusbar and two (or three) transducer fingers connected to the otherbusbar. The center distances between successive transducer fingers(irrespective of which polarity, that is to say including the fingerswithin a "split finger" and apart from points at which transducerfingers are omitted) are a quarter (or a sixth) of the acousticwavelength of the surface wave to which the relevant transducer istuned, to be precise the wavelength of the acoustic wave in thesubstrate material of the transducer. The center distances of adjacentgroups of two (or three) adjacent fingers of the same polarity, that isto say the center distances between alternating split fingers, is onceagain equal to half the wavelength, as in the case of simpletransducers.

A further transducer type with suppression of internal reflections ofthe ultrasound waves is the third-lambda finger arrangement. This islikewise an interdigital transducer in which, as in the case of thesplit finger transducer, the one finger comb (with the busbar) hasdoubled transducer fingers and there is in each case a single finger ofthe finger comb of the other busbar between adjacent groups of suchdouble transducer fingers.

For the internal regions of such a split finger transducer orthird-lambda transducer, the neutralization or suppression of internalreflections in the transducer is virtually complete with correspondinglyprecise dimensioning. However, this unfortunately does not apply to theterminal split finger groups and finger groups. In order to achievesuppression of reflections for these as well, it is known (Europeanreference EP-A-0,088,400) for the relevant transducers to be providedwith terminal transducer fingers which are dimensioned such that theirfinger length is reduced in steps, in such a manner that terminaltransducer fingers interact in an interfering manner with one anotherwith respect to the reflections. A disadvantage of such an (additional)measure is, however, that such an end of stepped construction of atransducer causes distortions in the phase front of the acoustic wave ofthe transducer. In addition, not all the fingers of a transducer whichis weighted in an overlapping manner, as is often the case, are affectedcompletely by the acoustic wave which is received by the transducerwhich is operated as the receiving transducer. The efficiency of such ameasure for suppressing undesired reflections occurring at the end of atransducer is also only limited.

Another method of suppressing reflections occurring on terminal fingersof an interdigital transducer is that which is described in Germanreference DE-B-2,839,851. Weighted fingers in the region of thetransducer end are directed with essentially longitudinal components ofthese fingers at an oblique angle with respect to the major axis of thewave propagation, so that these fingers reflect ultrasound waves whichimpinge on them away in an oblique direction, by means of theseobliquely positioned components of their finger lengths, that is to saysaid fingers do not intrinsically reflect back.

A further method (Trans. IECE Jap. Vol 64-C (1981) pages 437-438) is todesign some of the terminal fingers to be shortened with respect to themutual distance between busbars which are opposite one another, and toreplace the "missing" finger length by a floating finger in each case.However, floating potentials are highly problematic in manyapplications.

Finally, reference should also be made to a method of suppressingreflections of terminal fingers which consists of the outermost fingerof the two fingers of the terminal (consisting of these two fingers)split finger being designed to be directed obliquely with respect to themajor axis direction of wave propagation. With this measure, it must beaccepted that interference occurs in the phase front of the acousticwave of the useful signal.

Reference should also be made here to U.S. Pat. No. 4,162,465 which isapparently similar to the invention which is also described in thefollowing text. This document describes a split finger transducer inwhich, in contrast to split finger transducers which were known at thattime and in which wave reflections are suppressed in an artificialmanner at the fingers, such mechanical reflections are causeddeliberately. The purpose of this measure is to compensate for, that isto say to eliminate (reflected) waves which are reflected on theelectrical load by means of such deliberately caused waves. To this end,according to the teaching of this document, it is necessary, to beprecise within in each case one of the two transducer combs of theinterdigital transducer, to provide fingers with a finger width whichdeviates from the other fingers, and a deviating finger separation, oneor more such fingers of the one comb being adjacent to one or morefingers of the other comb. The mechanical (MEL) wave, which compensatesfor the wave which is reflected electrically (regenerated) by the otherload, is produced in the interior of the thus modified transducer. Thedimensioning, which for this purpose is dependent on the impedance ofthe load, of these deviating fingers is specified. This has nothing todo with the suppression of reflections of terminal fingers of atransducer and the fingers, dimensioned according to U.S. Pat. No.4,162,465, of the transducer described there are also not effective forthis purpose.

SUMMARY OF THE INVENTION

The object of the present invention is to specify measures by means ofwhich such reflection effects occurring at the ends of a transducer aresuppressed, to be precise without other known disadvantages associatedwith operation occurring.

This object is achieved by means of the configuration of a transducerwherein the transducer fingers, are arranged interdigitally adjacent tothe terminal fingers, are arranged corresponding to the specifiedacoustic wavelength of the transducer at intervals specified by a schemecorresponding to the specified design instruction, and are designed withfinger widths that are also specified by the scheme. On at least one endof the transducer are positioned, a plurality of terminal fingers whichare connected to only one busbar and that deviate from this scheme forthe positioning and the finger width of the other transducer fingers.They deviate in their finger strip width, such that the phases andamplitudes of the wave components, which would otherwise be reflected bythese terminal transducer fingers and by the adjacent final transducerfinger, which corresponds to the scheme of the transducer, are modifiedsuch that these wave components intrinsically interfere destructivelywith one another.

The following are advantageous developments of the present invention.Terminal transducer fingers dimensioned in the above described mannerare provided at both ends of the transducer. In the case of a transducerhaving terminal transducer fingers at one transducer end dimensioned asdescribed above, the other end of the transducer is provided withterminal fingers which are shortened in steps.

In the case of a transducer having terminal transducer fingers at onetransducer end dimensioned as described above, the other end of thetransducer is provided with terminal fingers which are directedobliquely with respect to a finger longitudinal part. In one embodimentthe last transducer finger which corresponds to the scheme of thetransducer is followed towards the transducer end by three terminaltransducer fingers, of which the third last transducer finger has asmaller finger width, the second last transducer finger has a largerfinger width, and the last, outermost transducer finger has again asmaller finger width, than a transducer finger which corresponds to thescheme of the transducer. Also, the intermediate spaces between the lasttransducer finger which corresponds to the scheme of the transducer andthe third last transducer finger, and the intermediate space between thesecond last transducer finger and the last outermost transducer fingerare larger than the intermediate space between transducer fingers of thescheme of the transducer. The number of terminal fingers which aredimensioned and positioned in a deviating manner at one transducer endis in the range of two to four.

The present invention is based on the knowledge that--with respect tothe normal scheme, which corresponds to the specified designinstruction, for the width and positions of the transducer fingers inthe interior of the transducer--by modifying the finger width and thefinger position of the last transducer finger at the relevant end of thetransducer, preferably the last two to four fingers, satisfactoryreflection suppression of reflections of terminal transducer fingers canbe achieved. This measure is primarily provided at the mutually oppositeends of two such transducers of a filter. In the case of the invention,the neutralization of reflections which nevertheless occur at thetransducer end despite a low-reflection finger arrangement of thetransducer fingers is thus effected by interference occurring as aresult of "incorrect positioning" and/or "incorrect dimensioning of thefinger width" of the last finger/fingers. The invention can be usedparticularly well in the case of transducers having single fingers (incontrast to split fingers).

The measure according to the invention avoids the known phase frontdistortions which occur, for example, in a design having fingers whichare shortened in steps. This design having fingers which are shortenedin steps or the other known measures, which have already been mentionedabove, for the suppression of reflections of terminal fingers cannot,however, on the other hand effect interference of the desiredtransmission quality of the filter at the averted end, that is to say atthe end of a transducer facing the sump.

The following information should be given with respect to thedimensioning according to the invention of these "incorrect positioning"and "finger width deviations":

The reflections occurring in a transducer having a structure, forexample a split finger structure, specified for the present applicationare calculated using methods from the prior art (IEEE Ultrason. Symp.Proc. (1985) pages 78-81) for interdigital transducers having fingerpositions, which are selected to be different in a specified manner, andfinger widths of terminal transducer fingers (for a given frequencyband). This is carried out with the aid of an iterative calculationprocess, using the least squares method. The iteration is terminated assoon as a geometry of four positionings or finger width deviations isachieved which results in satisfactorily low reflections (on theterminal transducer fingers). These are then the dimensions to be usedfor the individual case for the finger widths and for the incorrectpositionings of the selected terminal transducer fingers, that is to saythis produces the optimized deviations of dimensions and positions ofthe terminal transducer fingers with respect to the transducer fingersof the scheme of the central transducer fingers.

The relevant terminal transducer fingers are connected to one and thesame busbar which is connected, in particular, to ground. The measureaccording to the invention does not interfere with wave phase fronts,there are no floating potentials and such transducers or filtersconstructed according to the invention can be produced using the normaltechnological measures of lithography. The deviations for the fingerwidths are between up to virtually double and approximately a fifth to atenth of the finger widths of the transducer fingers of the scheme, thelithography determining the limit, for high frequencies, for the minimumwidth of a transducer finger. The (with respect to the scheme) selectedchanges in the finger intermediate spaces are in approximately the samevariation range. Numerical examples are intended to clarify the ordersof magnitude. A finger separation of 6.92 μm applies to a split fingertransducer for a center frequency of 125 MHz for finger metallizationsconsisting of 0.3 μm of aluminum for the transducer fingers of thescheme, that is to say for fingers which are not terminal fingers. Thesame dimension applies to the finger width. Terminal fingers designedaccording to the invention have the following dimensions:

    ______________________________________                                                    Finger width                                                                           Finger separation                                        ______________________________________                                        Last finger   0.70 μm 6.38 μm                                           Second last finger                                                                          4.33 μm 7.66 μm                                           Third last finger                                                                           0.70 μm 3.56 μm                                           ______________________________________                                    

With a finger width and finger separation of 6.59 μm for the fingers ofthe transducer scheme, the following figures result for the optimizedterminal transducer fingers of a transducer having a third-lambdastructure of a filter with a center frequency of 170 MHz and a 12%relative bandwidth:

    ______________________________________                                                    Finger width                                                                           Finger separation                                        ______________________________________                                        Last finger   4.20 μm 0.84 μm                                           Second last finger                                                                          2.89 μm 3.92 μm                                           Third last finger                                                                           6.96 μm 1.35 μm                                           ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawing, and in which:

FIG. 1 shows a surface wave filter having two transducers

FIG. 2 shows a first embodiment of a transducer according to theinvention and

FIG. 3 shows an embodiment which contains the invention and combines aknown measure.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically a surface wave filter 1 having a plate-shapedsubstrate 2 and having interdigital transducers 3 and 4 located on thesurface of the substrate. The double-headed arrow 5 refers to the axisof the major propagation direction of the acoustic waves, whichdirection is produced by the one of the two transducers as atransmitting transducer and is received by the other transducer as areceiving transducer.

FIG. 2 shows one of the transducers of the filter of FIG. 1, for exampletransducer 4. The transducer fingers F, which are electrically connectedin each case to the one or to the other busbar, can be seen between themutually opposite busbars 11 and 12. Transducer fingers corresponding tothe specified scheme of the transducer are designated by F₀. As can beseen from FIG. 2, these transducer fingers F₀ are positioned regularlywith respect to one another and have the same finger width. They fillthe central part of the transducer 4 and, in the exemplary embodiment ofFIG. 2, are arranged and distributed with respect to one anothercorresponding to a third-lambda transducer.

For completeness, it should be mentioned that these fingers F₀ may alsobe arranged at such separations which are different from one another andmay be designed to have correspondingly different fingers widths,specifically as is adequately known by the person skilled in the art fora dispersive transducer. A transducer of this embodiment also forms partof the invention.

The transducer 4 of FIG. 2 corresponding to the invention has terminaltransducer fingers F₁, F₂ and F₃ positioned at both ends and dimensionedwith respect to the finger width. It can clearly be seen from FIG. 2that both the finger widths and the distances between the fingers andthe distance between finger F₃ and the adjacent finger F₀ are clearlydimensioned differently, to be precise differently from one another anddifferently with respect to the positions and finger widths of thefinger F₀. This is the characteristic of the invention. The two fingersF₁ and F₂ are narrower than finger F₀ and are additionally at a distancefrom one another which is comparatively only very short. In contrast,the distance between the finger F₂ and the finger F₃ is greater, but isnevertheless smaller than the distance between the fingers F₀. Thefinger F₃ is broader than a finger F₀. The distance between the fingerF₃ and the adjacent finger F₀, that is to say the first finger F₀corresponding to the scheme of the transducer, is once again smallerthan the normal finger separation between adjacent fingers F₀.

The other end of the transducer 4 has the corresponding fingers F₁ ', F₂' and F₃ ', which are dimensioned and positioned according to theinvention.

The fingers F₁, F₂ and F₃ are preferably connected to one and the samebusbar 11, that is to say they are at the same potential, which isespecially ground potential.

FIG. 3 shows a transducer, which likewise corresponds to the invention,and is designated transducer 3. The transducer has fingers F₀ which arearranged and selected corresponding to a split finger transducer. Thefingers F₁₁, F₁₂ and F₁₃ are once again the terminal transducer fingerswhich are positioned according to the invention and are dimensioned withrespect to their finger width. In the case of transducer 3, theiteration method described above has resulted in a very narrow fingerF₁₁, a likewise very narrow finger F₁₃ and a finger F₁₂ which incontrast is broader, the last finger nevertheless being narrower than afinger F₀. The separations of the fingers F₁₁, F₁₂, F₁₃ and of thefinger F₀ from the adjacent finger 13, which are from the regular fingerseparations of the fingers F₀, can also be seen.

The end 31 of the transducer 3 located opposite this configurationaccording to the invention is constructed with fingers which areshortened in steps, in accordance with the abovementioned known priorart, instead of corresponding to the invention. If the transducers ofFIGS. 2 and 3, as they are aligned in their figures, are located in thefilter 1 of FIG. 1, the end of the transducer 3 with its fingers F₁₁,F₁₂ and F₁₃, and the end of the transducer 4 with its fingers F₁, F₂,and F₃ are located opposite one another. The reflections which wouldotherwise occur on terminal fingers of the interdigital converter do notoccur here according to the invention, or are greatly suppressed.Because the fingers F₁, F₂, F₃ and F₁₁, F₁₂, F₁₃ are aligned in a commonmanner with respect to the axis 5, there are no disadvantageousinfluences on the wave front of the acoustic wave which occurs betweenthe transducers 3 and 4.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A surface wave transducer having measures forsuppressing internal reflections on terminal fingers of a transducerhaving other transducer fingers, which are arranged interdigitallyadjacent to said terminal fingers, are arranged corresponding to aspecified acoustic wavelength of the transducer at intervals specifiedby a predetermined scheme, and are designed with finger widths that arelikewise specified by the predetermined scheme, comprising: terminalfingers on each end of the transducer and on each end of the transducer,only a plurality of terminal fingers which are connected to one busbarof the transducer, are positioned deviating from said scheme forpositionings and finger widths of the other transducer fingers and arepositioned deviating in their finger strip width, such that phases andamplitudes of wave components, which would otherwise be reflected bysaid terminal transducer fingers and by an adjacent final transducerfinger of the other transducer fingers, which corresponds to the schemeof the transducer, are modified such that said wave componentsintrinsically interfere destructively with one another.
 2. A surfacewave transducer having measures for suppressing internal reflections onterminal fingers of a transducer having other transducer fingers, whichare arranged interdigitally adjacent to said terminal fingers, arearranged corresponding to a specified acoustic wavelength of thetransducer at intervals specified by a predetermined scheme, and aredesigned with finger widths that are likewise specified by thepredetermined scheme, comprising: on at least one end of the transducer,only a plurality of terminal fingers which are connected to one busbarof the transducer, are positioned deviating from said scheme forpositionings and finger widths of the other transducer fingers and arepositioned deviating in their finger strip width, such that phases andamplitudes of wave components, which would otherwise be reflected bysaid terminal transducer fingers and by and adjacent final transducerfinger of the other transducer fingers which corresponds to the schemeof the transducer, are modified such that said wave componentsintrinsically interfere destructively with one another, the finaltransducer finger which corresponds to the scheme of the transducerbeing followed towards the end of the transducer by three terminaltransducer fingers, of which a third last transducer finger has asmaller finger width, a second last transducer finger has a largerfinger width, and a last, outermost transducer finger has a smallerfinger width again, that a transducer finger which corresponds to thescheme of the transducer and an intermediate space between the lasttransducer and the third last transducer finger, and an intermediatespace between the second last transducer finger and the last outermosttransducer finger being larger than an intermediate space between theother transducer fingers of the scheme of the transducer.